Radio-frequency transceivers, as the one shown in FIG. 1, include a transmitter TX and a receiver RX of a radio-frequency signal RF modulated on a carrier LO. The receiver circuit should be capable of amplifying and converting to base-band (BB) the fable antenna signal without excessively degrading its noise-to-signal ratio (S/N).
The first amplification stage—commonly called a Low Noise Amplifier (LNA)—is particularly critical because, besides properly amplifying the input radio-frequency signal, it is desired that it satisfy specifications of impedance matching with the antenna or eventual filters, and degrading as less as possible the input S/N ratio.
Another restraint of great importance, especially in portable devices, is tied to the limited availability of energy for powering the system, which typically implies a limitation of available current for powering the functional blocks, including the LNA. Typically, attendant to a low consumption design is a reduction of the gain of the functional circuits and an increased influence of internally generated noise.
Moreover, low noise amplifiers disclosed in literature are designed for satisfying gain specifications, noise and impedance matching on relatively narrow bands. Typically, they use a certain number of reactive components, that are often external components. This makes these known circuits scarcely flexible, relatively encumbering and expensive.
The need of low noise circuits strongly limits complexity of topologies and the number of components that may be used to make the LNA. For this reason, numerous approaches disclosed in literature are based on common source (CS) or common gate (CG) amplification stages. Typically, CS configurations have a comparably better noise performance and implement the desired impedance matching by using reactive components (inductors and capacitors) that typically lead to a narrow band circuit, hardly realizable in completely integrated form.
CG architectures, though in general have a poorer noise performance than CS architectures, may be used over broader frequency bands because the amplification stage has an input impedance with a finite real part and a minimum noise figure Fmin that are both independent from the working frequency.
Another common gate amplification stage is illustrated in FIG. 2. Vin is the input signal that is effectively amplified, Vsig is the signal captured by the antenna, and RS the internal resistance of the antenna. By neglecting the effects of the input capacitance, the maximum power transfer from the antenna to the amplification stage is for rin=RS wherein rin is the real part of the input impedance of the antenna Zin. Given that:
            r      in        ≈                  1                  g                      M            ⁢                                                  ⁢            1                              ⁢                          ⁢      and      ⁢                          ⁢              g                  M          ⁢                                          ⁢          1                      =      2    ⁢                  k        ⁢                  W          L                ⁢                  I                      B            ⁢                                                  ⁢            1                              _      wherein gM1 is the transconductance of the transistor M1 and W/L is the aspect ratio, this condition is relatively restrictive, especially for a low consumption design.
If the bias current IB1 is limited to a value of about one mA and if RS=50Ω, the condition rin=RS is practically not realizable because the value of gM1 that would satisfy this condition is not compatible with the level of IB1 and with aspect ratios W/L of the transistors adapted to the common working frequencies. Indeed, a large size of the transistor M1 implies a high capacitance, thus the reactive (capacitive) part Zin would no longer be negligible.
A known architecture that partially obviates to the issues connected with a large aspect ratio W/L of M1, in case of a limited bias current IB1, is shown in FIG. 3. The circuit includes an inductor LS that compensates the capacitive portion of the input impedance Zin, and a reactive network MATCHING NETWORK for matching the input impedance rin≈1/gM1 (typically of the order of several hundreds of Ohms) with the resistance RS.
This approach, besides using hardly integrable inductive components or components of poor quality, that make it more expensive and less practical, jeopardizes the capability of functioning, in the desired manner over a broad band, which is the characteristic of common gate amplifiers.